1. Field of the Invention
The invention relates to a field emission display (FED) technology, and more particularly to a triode structure with a gate layer and a cathode layer patterned on the same plane during the same process. The triode structure uses the gate layer to pull out electrons from lateral cathode layers, resulting in high luminescent efficiency.
2. Description of the Related Art
Field emission display (FED), a competing technology in the panel display market, is a high-voltage display with a triode structure consisting of anode, cathode, and gate electrodes to achieve high illumination by applying a high voltage and a low current. FED has advantages of light weight and thin profile, like liquid crystal display (LCD), and advantages of high brightness and self luminescence, like cathode ray tube (CRT). In a conventional triode structure of FED, the anode is used to increase energy of electrons, the cathode is used to emit electrons and the gate electrode is used to pull electrons out from the cathode, thus the triode structure can increase luminescent efficiency and reduce controlled voltage. With regard to the fabrication of an electron-emitting source, molybdenum (Mo) metal is employed to form a micro-tip shape, despite the attendant problems of complex process, expensive equipment cost, and low throughput. Recently, carbon nanotubes (CNTs), having highs mechanical strength and great electrical performance, have been coated/grown within an electron-emitting area as an electron-emitting source, resulting in a CNT-FED device.
FIG. 1 is a sectional diagram showing a conventional CNT-FED device. The CNT-FED device 10 has a cathode substrate 12, an anode substrate 14 over and parallel to the cathode substrate 12, and a spacer disposed in the vacuum space between the two substrates 12 and 14 for maintaining a predetermined vertical distance and resisting atmospheric pressure. Generally, the two substrates 12 and 14 are glass substrates. The anode substrate 14 has a plurality of transverse-extending anode layers 16 of ITO, a black matrix layer 18, a plurality of fluorescent layers 20 and planarized Al film 22. The fluorescent layers 20 consist of a red layer 20R, a green layer 20G and a blue layer 20B. The Al film 22 is employed as a conductive layer of the anode substrate 14, a reflective layer of the fluorescent layer 20 and a protective layer for protecting the fluorescent layer 20 from ion bombardment and electric-field attraction. The cathode substrate 12 has a plurality of lengthwise-extending cathode layers 24, a plurality of CNT emitting layers 26 formed on each electron-emitting area of the cathode layer 24, an insulating layer 28 formed on peripheral region of each electron-emitting area for isolating adjacent CNT emitting layers 26, and a gate electrode layer 29 patterned on the insulating layer 28.
In one method of forming the CNT emitting layer 26, the CNT material is formed within the electron emitting area prior to deposition, sintering and etching for the formation of the insulating layer 28 and the gate electrode layer 29. However, those processes consisting of deposition, sintering and etching may deteriorate the CNT, resulting in unstable emission. In another method of forming the CNT emitting layer 26, the insulating layer 28 and the gate electrode layer 29 are formed to provide an opening corresponding to the electron emitting area, and then the opening is filled with the CNT material. However, this easily causes a short circuit between the gate electrode layer 29 and the cathode layer 24, and it is difficult to accurately control the opening depth for filling the CNT material and the uniformity of the CNT material on the electron emitting area.
Accordingly, a reflective-type electrode and an under-gated structure have been developed to simplify the FED process and achieve the same characteristics provided by the above-described triode structure.
FIG. 2A is a reflective-type electrode structure of a conventional CNT-FED device. FIG. 2B is a sectional diagram of a pixel unit of the reflective-type electrode structure. A reflective-type triode structure 30 comprises a bottom glass substrate 32 and an upper glass substrate. The bottom glass substrate 32 comprises a plurality of transverse-extending anode layers 34, a plurality of transverse-extending fluorescent layers 36R, 36G and 36B, a plurality of lengthwise-extending dielectric layers 38, a plurality of lengthwise-extending cathode layers 40 and a plurality of CNT emitting layers 42 arranged in a matrix. The upper glass substrate comprises a transparent conductive layer 44. In a pixel unit, the anode layer 34 provides an anode electrical field to pull electrons out of the cathode layer 40 by a lateral force. Meanwhile, the transparent conductive layer 44 provides a cathode electrical field to push electrons downward. Thus, the anode voltage and the cathode voltage between the two substrates 44 and 32 can gather an electron beam and the electrons precisely bombard the fluorescent layer 36, resulting in luminescence.
The reflective-type electrode structure 30 has a simplified process and stable emitting property because the CNT emitting layer 42 can be formed during the last procedure without suffering damage from the subsequent processes. Also, a surface treatment can be further performed on the CNT emitting layer 42 to improve electron emitting characteristics thereof. However, limited to driving circuits for the reflective-type structure 30, the anode voltage is 2˜300V that is insufficient for high luminescence. Moreover, since the control of the anode voltage and the cathode voltage is complex, it is difficult to gather the electron beam.
FIG. 3A is a solid diagram showing an under-gate structure of a conventional CNT-FED device. FIG. 3B is a sectional diagram of an under-gate structure of a conventional CNT-FED device. AN under-gate structure 50 comprises a lower glass substrate 52 and an upper glass substrate 64. The lower glass substrate 52 comprises a plurality of transverse-extending counter electrode layers 54, an insulating layer 55, a plurality of under-gate layers 56 arranged in a matrix, a plurality of lengthwise-extending cathode layers 58 and a plurality of lengthwise-extending CNT emitting layers 60. The upper glass substrate 62 comprises a plurality of transverse-extending anode layers 64 and a plurality of transverse-extending fluorescent layers 66. In the under-gate structure 50, electrons are pulled out from the CNT emitting layer 60 by the under-gate layer 56 and are then sped by a voltage of the anode layer 64 to bombard the fluorescent layer 66.
The under-gate structure 50 has the same advantages as the reflective-type structure 30 despite the attendant disadvantages as follow. First, the voltage of the anode layer 64 must be precisely controlled to ensure that the electron beam bombard an appropriate position. Second, in order to stop luminance, a negative voltage should be provided by the under-gate layer 56 to restrain electrons from emission, thus an extra control voltage level is needed. Third, in order to prevent the cross-talk effect between the under-gate layer 56 and the cathode layer 58, the interval between the two adjacent cathode layers 58 should be larger to increase the space between the under-gate layer 56 and the cathode layer 58.